Design, synthesis and evaluation of 3-hydroxypyridin-4-ones as inhibitors of catechol-O-methyltransferase

Parkinson’s disease is a bradykinetic disorder that is the result of the death of dopaminergic neurons in the basal ganglia of the brain. This, in turn, leads to the depletion of dopamine in the striatum, which is responsible for the characteristic motor symptoms of the disease. The most effective treatment for restoring central dopamine levels is levodopa, the metabolic precursor of dopamine. However, due to extensive peripheral enzymatic metabolism by dopa decarboxylase (DDC) and O-methylation by catechol-O-methyltransferase (COMT), less than 1% of levodopa reaches the brain unchanged. Thus, by preventing levodopa metabolism and increasing the availability of levodopa for uptake into the brain, the inhibition of COMT would be beneficial in Parkinson’s disease. COMT serves as a catalyst in the methyl transmission from S-adenosyl-L-methionine (SAMe/SAM) to a hydroxy group of a catechol substrate. Nitrocatechol COMT inhibitors, such as tolcapone and entacapone, have been used in the treatment of Parkinson’s disease. Poor bioavailability and undesirable side-effect profiles sometimes limit the clinical use of nitrocatechol COMT inhibitors. The aim of this study therefore was to discover new non-nitrocatechol COMT inhibitors for the treatment of Parkinson’s disease. In the present study, the 3-hydroxypyridin-4-one scaffold was selected for the design of non-nitrocatechol COMT inhibitors since the COMT inhibitory potential of this class has been illustrated. 3-Hydroxypyridin-4-ones are isosteric to the catechol ring, but are not O-methylated by the enzyme themselves. Further, it has been illustrated that non-nitrocatechol COMT inhibitors can be MB-COMT (membrane bound COMT) specific, which may be beneficial when considering peripheral side-effects. The present study thus reports the synthesis of new members of the 3-hydroxypyridin-4-one class of compounds, which may act as COMT inhibitors. Such compounds may represent useful agents for the treatment of Parkinson’s disease with improved safety profiles compared to nitrocatechol COMT inhibitors. Different structural aspects of the nitrogen substituent that were explored included simple aromatic and aliphatic substitution (JDB1, JDB10 and JDB11), chain elongation and increasing flexibility (JDB3, JDB4, JDB5 and JDB9), as well as halogen and methyl substitution of the side chain phenyl ring (JDB12, JDB13 and JDB14). Molecular modelling studies (Discovery Studio 3.1, Accelrys) were conducted in a preliminary attempt to predict the inhibition activities of the proposed 3-hydroxypyridin-4-one derivatives. All the derivatives fitted within the catechol binding site of COMT and formed productive interactions with the residues of the enzyme. Therefore the potential inhibition activities of these compounds were confirmed. The compounds were synthesised by reacting maltol with a suitable primary amine in an acidic environment, with ethanol serving as co-solvent. Nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (IR) and mass spectrometry (MS) were used to characterise the structures. The purities of the compounds were estimated by high-performance liquid chromatography (HPLC) analysis. COMT obtained from porcine liver was used as enzyme source to evaluate the in vitro COMT inhibitory properties of the synthesised 3-hydroxypyridin-4-ones. A HPLC method with fluorescence detection was validated and employed to measure COMT activity. The natural COMT substrate, (-)-norepinephrine (NE), was incubated with COMT in the presence of various concentrations of the test inhibitors. The formation of normetanephrine (NMN), the O-methylated product of NE metabolism, was measured by the validated HPLC system. From the inhibition data IC50 (inhibitor concentration at 50% inhibition) values for the inhibition of COMT were calculated. The synthesised 3-hydroxypyridin-4-ones were found to be inhibitors of COMT with IC50 values ranging from 4.55 to 19.79 μM. Compared to the reference COMT inhibitors, entacapone (IC50 = 0.00047 μM) and tolcapone (IC50 = 0.00675 μM), the 3-hydroxypyridin-4-ones were significantly lower potency COMT inhibitors. 1-Benzyl-3-hydroxy-2-methylpyridin-4-one (JDB3) was the most potent compound with an IC50 value of 4.55 μM. Some preliminary structure-activity relationships (SARs) were derived, for example, benzyl substitution of the 3-hydroxypyridin-4-one moiety yielded the most potent COMT inhibitors of the series. Phenylethyl, phenylpropyl and phenylbutyl substitution yielded lower potency inhibitors. This shows that chain elongation of the substituent reduces COMT inhibition potency. In conclusion, several 3-hydroxypyridin-4-one derivatives were synthesised and their COMT inhibitory activities were determined. Although these compounds are not highly potent inhibitors, they may act as leads for the development of non-nitrocatechol COMT inhibitors with possibly better safety profiles. Such compounds would be appropriate for the treatment of Parkinson’s disease.National Research Foundatio